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Operational challenges of the LHCOperational challenges of the LHCJörg Wenninger

CERN Accelerators and Beams Department Operations group

November 2007

Part 1: LHC overview and status•LHC overview•LHC magnets•Hardware & beam commissioning

Outline

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• LHC overview

• LHC magnet system

• LHC commissioning

• Luminosity

• LHC injector chain

• Machine protection

• Collimation

Part 1

Part 2

LHC History

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1982 : First studies for the LHC project1983 : Z0/W discovered at SPS proton antiproton collider (SppbarS) 1989 : Start of LEP operation (Z boson-factory)1994 : Approval of the LHC by the CERN Council1996 : Final decision to start the LHC construction 1996 : LEP operation > 80 GeV (W boson -factory)2000 : Last year of LEP operation above 100 GeV2002 : LEP equipment removed2003 : Start of the LHC installation2005 : Start of LHC hardware commissioning2007 : First sector at 1.9K, all magnets at 1 TeV.2008+ : Expected LHC commissioning with beam

4Rüdiger Schmidt Bullay Oktober 2007 4

The CERN Beschleuniger Komplex

CERN (Meyrin)

LHC

SPSControl room

ATLAS

LHCBCMS

ALICE

7 years of construction to replace

LEP: 1989-2000

in the same 26.7 km tunnel by

LHC : 2008-2020+

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Tunnel circumference 26.7 km, tunnel diameter 3.8 mDepth : ~ 70-140 m – tunnel is inclined by ~ 1.4%

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IR6: Beam dumping system

IR4: RF + Beam instrumentation

IR5:CMS

IR1: ATLAS

IR8: LHC-BIR2:ALICE

Injection ring 2Injection ring 1

IR3: Momentum collimation (normal

conducting magnets)

IR7: Betatron collimation (normal

conducting magnets)

Beam dump blocks

LHC Layout8 arcs. 8 long straight sections (insertions), ~ 700 m long.beam 1 : clockwisebeam 2 : counter-clockwiseThe beams exchange their positions (inside/outside) in 4 points to ensure that both rings have the same circumference !

The main dipole magnets define the

geometry of the circle !

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Top energy/GeV Circumference/m Linac 0.12 30PSB 1.4 157CPS 26 628 = 4 PSBSPS 450 6’911 = 11 x PSLHC 7000 26’657 = 27/7 x SPSLEIR

CPS

SPS

Booster

LINACS

LHC

3

45

6

7

8

1

2

Ions

protons

Beam 1

Beam 2

TI8

TI2

Note the energy gain/machine of 10 to 20 – and not more !The gain is typical for the useful range of magnets !!!

In total > 50 km of beam lines

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LHC – yet another collider?The LHC surpasses existing accelerators/colliders in 2 aspects :

The energy of the beam of 7 TeV that is achieved within the size constraints of the existing 26.7 km LEP tunnel.

LHC dipole field 8.3 THERA/Tevatron ~ 4 T

The luminosity of the collider that will reach unprecedented values for a hadron machine:

LHC pp ~ 1034 cm-2 s-1

Tevatron pp 2x1032 cm-2 s-1

SppbarS pp 6x1030 cm-2 s-1

The combination of very high field magnets and very high beam intensities required to reach the luminosity targets makes operation of the LHC a great challenge !

A factor 2 in field

A factor 4 in size

A factor 100 in luminosity

Field challenges

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The force on a charged particle is given by the Lorentz force which is proportional to the charge, and to the vector product of velocity and magnetic field:

)( BvEF

q

To reach a momentum of 7 TeV/c given the LHC (LEP) bending radius of 2805 m:

Bending field B = 8.33 Tesla Superconducting magnets Re

pB

0

y

x

s

v

B

F

To collide two counter-rotating proton beams, the beams must be in separate vaccum chambers (in the bending sections) with opposite B field direction.

There are actually 2 LHCs and the magnets have a 2-magnets-in-one design!

Luminosity challenges

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The event rate N for a physics process with cross-section is proprotional to the collider Luminosity L:

To maximize L:

• Many bunches (k)

• Many protons per bunch (N)

• A small beam size u = ()1/2

: characterizes the beam envelope (optics), varies along the ring, mim. at the collision points. : is the phase space volume occupied by the beam (constant along the ring).

**

2

4 yx

fkNL

k = number of bunches = 2808N = no. protons per bunch = 1.15×1011

f = revolution frequency = 11.25 kHzx,y = beam sizes at collision point (hor./vert.) = 16 m

LN

High beam “brillance” N/

(particles per phase space

volume)

Injector chain performance !

Small envelope

Strong focusing !

The price of high fields & high luminosity…

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When the LHC is operated at 7 TeV with its design luminosity & intensity,

the LHC magnets store a huge amount of energy in their magnetic fields:per dipole magnet Estored = 7 MJall magnets Estored = 10.4 GJ

the 2808 LHC bunches store a large amount of kinetic energy:Ebunch = N x E = 1.15 x 1011 x 7 TeV = 129 kJEbeam = k x Ebunch = 2808 x Ebunch = 362 MJ

To ensure safe operation will be a major challenge for the LHC operation crews !

Protection of the machine components from the beam is becoming a major issues for all new high power machines (LHC, SNS, …).

> 1000 x above damage limit for accelerator

components !

Stored Energy

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Increase with respect to existing accelerators :• A factor 2 in magnetic field• A factor 7 in beam energy• A factor 200 in stored energy

Comparison…

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The energy of an A380 at 700 km/hour corresponds

to the energy stored in the LHC magnet system :

Sufficient to heat up and melt 12 tons of Copper!!

The energy stored in one LHC beam corresponds

approximately to…

• 90 kg of TNT

• 8 litres of gasoline

• 15 kg of chocolate

It’s how ease the energy is released that matters most !!

LHC Magnets

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Superconductivity

15Temperature [K]

App

lied

field

[T]

Superconductingstate

Normal state

Bc

Tc

The very high DIPOLE field of 8.3 Tesla required to achieve 7 TeV/c can only be obtained with superconducting magnets !

The material determines:Tc critical temperature Bc critical field

The cable production determines: Jc critical current density

Lower temperature increased current density higher fields.

Typical for NbTi @ 4.2 K 2000 A/mm2 @ 6T

To reach 8-10 T, the temperature must be lowered to 1.9 K – superfluid Helium !

The superconducting cable

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1 mm6 m

Typical value for operation at 8T and 1.9 K: 800 A

Rutherford cable

width 15 mm

A.Verweij

A.Verweij

Coils for dipoles

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Dipole length 15 m

The coils must be aligned very precisely to ensure a

good field quality (i.e. ‘pure’ dipole)

Dipole field map - cross-section

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Superconducting coil

Beam

Iron

Non-magnetic collars

B = 8.33 Tesla I = 11800 A L = 0.1 H

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Beam tube

Superconducting coil

Non-magnetic collars

Ferromagnetic iron

Steel cylinder for Helium

Insulation vacuum

Supports

Vacuum tank

Weight (magnet + cryostat) ~ 30 tons, Length 15 m

Dipole magnet production challenges

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The field quality must be excellent:- Relative field errors due to multipoles much less than 0.1 %.- The coils/collars must be positioned to some 10 m.

The geometry must be respected – and the magnet must be correctly bent (‘banana’ shape) to follow the curvature of the trajectory.

All magnets had to be produced in time, delivered to CERN, installed in the cryostats, cold tested, and finally installed into the LHC tunnel.

The magnets must reach a field of at least 8.3 Tesla, and possibly 9 Tesla.

First dipole lowered on 7 March 2005

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Only one access point for 15 m long dipoles, 35 tons each

LHC arc lattice : not just dipoles

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Dipole- und Quadrupol magnets– Provide a stable trajectory for particles with nominal momentum.

Sextupole magnets– Correct the trajectories for off momentum particles (‚chromatic‘ errors).

Multipole-corrector magnets– Sextupole - and decapole corrector magnets at end of dipoles– Used to compensate field imperfections if the dipole magnets. To stabilize trajectories

for particles at larger amplitudes – beam lifetime !

~ 8000 superconducting magnets ae installed in the LHC

QF QD QFdipolemagnets

small sextupolecorrector magnets

decapolemagnets

LHC Cell - Length about 110 m (schematic layout)

sextupolemagnets

23J. Wenninger - ETHZ - December 2005 23

1232 main dipoles +

3700 multipole corrector magnets

(sextupole, octupole, decapole)

392 main quadrupoles +

2500 corrector magnets (dipole, sextupole, octupole)

Regular arc:

Magnets

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Regular arc:

Cryogenics

Supply and recovery of helium with 26 km long cryogenic distribution line

Static bath of superfluid helium at 1.9 K in cooling loops of 110 m length

Connection via service module and jumper

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Insulation vacuum for the cryogenic distribution line

Regular arc:

Vacuum

Insulation vacuum for the magnet cryostats

Beam vacuum for

Beam 1 + Beam 2

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Regular arc:

Electronics

Along the arc about several thousand electronic crates (radiation tolerant) for:

quench protection, power converters for orbit correctors and instrumentation (beam, vacuum + cryogenics)

Tunnel view

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Complex interconnects

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Many complex connections of super-conducting cable that will be buried in a cryostat once the work is finished.

This SC cable carries 12’000 Afor the main dipoles

Vacuum chamber

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Beamenvel. ~ 1.8 mm @ 7 TeV

50 mm

36 mm

The beams circulate in two ultra-high vacuum chambers made of Copper that are cooled to T = 4-20 K.

A beam screen protects the bore of the magnet from image currents, synchrotron light etc from the beam.

Cooling channel (Helium)

Beam screen

Magnet bore

Operational margin of SC magnet

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Temperature [K]

App

lied

field

[T]

Superconductingstate

Normal state

Bc

Tc

9 K

Applied Field [T] Bc critical field

1.9 K

quench with fast loss of ~1010 p ~ 0.01% of total int.

Quench with fast loss of ~106-7 p ~0.01-0.1 ppm of the total int.

8.3 T / 7 TeV

0.54 T / 450 GeV

QUENCH

Tc critical temperature

Temperature [K]

The LHC is ~1000 times more critical than

TEVATRON, HERA, RHIC

‘Cohabitation’

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Even to reach a luminosity of 1033 cm-2 s-1, i.e. 10% of the design, requires unprecedented amounts of stored beam energy.

The stored energy will be circulating a few cms from extremely sensitive super-conducting magnets, which can quench following a fast loss of less than 1 part per million of the beam:

>>> requires a very large collimation system (> 100 collimators).

LHC commissioning has to be much more rigorous than what was done for previous machines – no shortcuts and dirty ‘tricks’ can be used to go ahead as soon as the intensities exceed a few % of the design.

>>> Predictions on the commissioning duration are particularly tricky!

LHC Harware Commissioning

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LHC Commissioning

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Commissioning of the LHC equipment (‘Hardware commissioning’) has started in 2005 and is now in full progress. This phase includes:

Testing of ~10000 magnets (most of them superconducting). 27 km of cryogenic distribution line (QRL). 4 vacuum systems, each 27 km long. > 1600 magnet circuits with their power converters (60 A to 13000 kA). Protection systems for magnets and power converters. Checkout of beam monitoring devices. Etc…

Hardware commissioning sequence

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The present LHC commissioning phase is designated as ‘Hardware Commissioning’. The main job is the commissioning of the magnets and their powering systems.

When all magnets of one of the eight LHC sectors installed and interconnected: Pumping vacuum system to nominal pressure. Cooling down to 1.9 (4.5) Kelvin. Connection of the power converter to the magnets. Commissioning of the power converter + interlock system + magnet protection

system (low current). Commissioning of magnet powering + magnet protection system (high current). Powering of all magnets in a sector to nominal current.

This commissioning sequence is run individually on each of the 8 LHC sectors (arcs).

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LHC sector 78 - First cooldown

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15/01/0718:00

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Time (UTC)

Tem

per

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K)

Supply temperature Magnet temperature (average over sector)

Return temperature Thermal shields temp. (average over sector)

Active cooling in all thermal shields and

cryogenic tranfer lines. All magnets isolated.

Thermal shield

temp.

Supply

temperature

Return

temperature

Magnet

temp.

Re-start of active cooling for cryogenic

transfer lines

Re-start of active cooling for magnets

Active cooling in all thermal shields and

cryogenic tranfer lines. All magnets isolated.

LHC Sector 78 – First cooldown

From 300K to 80K pre-cooling with 1200 tons of liquid N2 (60 trucks !). Three weeks for first sector.

From 80K to 4.5K. Cool-down with liquid He refrigerators. Three weeks for the first sector. 4700 tons of material to be cooled.

From 4.5K to 1.9K. Cold compressors at 15 mbar. Four days for the first sector.

He inventory : 100 tons (entire LHC).

Finally – June 2007

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Commissioning status

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Magnet production is completed. Installation and interconnections finished for magnets. Some components still

missing (collimators…). Cryogenic system :

- One sector (IR8IR7) was cooled down to 1.9 K in June/July 2007.- Cool-down of 4 (1/2 LHC) sectors starting/in progress (until Christmas).

Powering system:- Power converter commissioning finished.- Cool-down and commissioning of the first complete sector (7-8) was performed in

June/July 2007:>> All circuits with individual magnets have been commissioned (except triplets). >> The main magnets were commissioned to 1 TeV: limited by electrical non-

conformities that have been repaired during a warm up of the sector September-October.

- Main commissioning campaign starts in December 2007. Other systems (RF, beam injection and extraction, beam instrumentation,

collimation, interlocks, etc) are essentially on schedule for first beam in 2008.

Commissioning problems…

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Given that: The LHC is of unprecedented complexity, The LHC performance/technology is pushed to the limits,

it is not really surprising that the history of the LHC is filled with more or less severe problems that were related to dipole magnets, cryogenics distribution line, collimators…

Two recent problems that concern the machine commissioning: ‘Inner triplets’ RF fingers

The (inner) ‘Triplets’

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distance about 100 m

Interaction point

QD QD QF QD QF QD

Experiment

The large aperture quadrupoles called ‘inner triplets’ are high gradient and large aperture magnets that provide the focussing for the collision point in both planes. They are provided as part of the US & JAPAN contributions to CERN :

- Large beam size ~ 100 x size at IP- Large beam separation from crossing angle ~ 12 mm

Beam sizes : - at IP (ATLAS, CMS) 16 m- in the triplets ~1.6 mm- in the arcs ~0.2 mm

Triplets viewed from LHC tunnel

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Triplet viewed from the CMS cavern

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Triplet problems

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In February 2007 a triplet magnet in point 5 was damaged during a (routine) pressure test. The support that holds the magnet in the cryostat could not sustain the longitudinal force during the pressure test.

A crash programme was initiated in collaboration with FNAL to repair the magnets, partly in situ.

All magnets are now repaired.

The faulty support

‘RF fingers’ problems

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RF bellows are used to maintain electrical contact between adjacent pieces of vacuum chamber (essential for beam stability).

The bellows must cope with the thermal expansion of ~ 4 cm between 1.9 K and room temperature (when the magnets are cooled down/warmed up).

Bellows are installed at every interconnection (1700 in total).

At room temperature

At operating temperature

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Damaged bellows

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RF fingers that bend into the beam are a classic problem for accelerators. RHIC suffered a lot from it…

So no excuses for not being careful in design and manufacturing ! And yet it happened ! X-ray imaging of some bellows revealed bend fingers in the

sector that was tested in July and warmed up since then for repair.

Cause & solution for RF fingers

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The fingers bend due to a combination of a wrong ‘finger angle’ PLUS a gap between the magnet apertures larger than nominal (still inside specification).

Only few interconnects are affected. Complete survey of one sector was performed using X-ray techniques. Repair is not difficult…once the bad fingers are found.

>> This problem will stay around until every sector is warmed up again in 200X…

‘Solution’: A small (ping-pong size) ball equipped with

a 40MHz transmitter is blown through the beam vacuum pipe with compressed air.

Beam position monitors are used to follow the ball as it rolls inside the vacuum chamber until it stops or exits on the other end..

Towards First Beam

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Injector status : beam at the gate to the LHC

49L.R. Evans – EDMS Document No. 521217

SPC

2

TI 8 commissioning

TI 8 commissioning / V.Mertens / TCC, 29.10.2004

First shot on BTVI87751 on 23 October 2004, 13:39

TV screen at end transfer line

Beam image taken less than 50 m away from the

LHC tunnel in IR8 (LHCb) !

The LHC injectors are ready after a long battle to achieve the nominal beam brightness: instabilities, e-clouds etc.

The nominal LHC beam can be produced at 450 GeV in the SPS.

Latest ‘schedule’

50R.Bailey, November 2007

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2007

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12 23 34 45 56 67 78 81

Machine Checkout

Beam Commissioning to 7 TeV

Consolidatio

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Interconnection of the continuous cryostat

Leak tests of the last sub-sectors

Inner Triplets repairs & interconnectionsGlobal pressure test &Consolidation

Flushing

Cool-downWarm up

Powering Tests

Global pressure test &ConsolidationCool-down

Powering Tests

General schedule Baseline rev. 4.0

Pushed to the limit – no contingency ! Expect changes … !!!!!!

Towards beam…

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The latest schedule is VERY aggressive: Assumes parallel commissioning of almost the entire machine, while the initial schedules assumed parallel commissioning of only 1/4 LHC in

parallel.

There is no contingency for problems. It is likely that at least one of the 7 sectors that have never been cooled down will reveal some problem(s) that require a warm up before

they can be fully commissioned to 7 TeV.

Some scenarios for 2008 that are under discussion assume:- Push hardware commissioning as far as possible without repairs.- If one (or more) sectors cannot be run at 7 TeV and require a warm up for

repair, start with beam commissioning at 450 GeV to gain experience before performing the repair.

Beam commissioning

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Parameter Phase A Phase B Phase C Nominalk / no. bunches 43-156 936 2808 2808

Bunch spacing (ns) 2021-566 75 25 25

N (1011 protons) 0.4-0.9 0.4-0.9 0.5 1.15

Crossing angle (rad) 0 250 280 280

*/*nom) 2 1 1

* (m, IR1&5) 32 22 16 16

L (cm-2s-1) 6x1030-1032 1032-1033 (1-2)x1033 1034

Beam commissioning will proceed in phases with increased complexity: Number of bunches and bunch intensity. Crossing angle (start without crossing angle !). Less focusing at the collision point (larger ‘*’).

It will most likely take YEARS to reach design luminosity !!!

Summary

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We are getting there at last !!!!• After many years of delay the LHC is now really taking shape in the tunnel.• A first 1/8th of the LHC was tested to 1 TeV last summer, expected to

reach 7 TeV during the winter.• The other 7/8th will be commissioned during the coming winter/spring.

Beam is knocking at the door : • The injectors are ready.• It is now very likely that beam will be injected into the LHC in 2008.• But the road to 7 TeV collisions is long, and it is difficult to predict when

the experiments will see first collisions at 7 TeV, or high luminosity !!